The present invention relates to a modulator and a modulator-integrated semiconductor laser device, and a Manufacturing Process therefor, for use in the optical communication. In particular, the present invention relates to the modulator and the modulator-integrated semiconductor laser device with an improvement in its modulating frequency characteristics by reducing a wire capacitance of the modulator, and the Manufacturing Process therefor.
In order to expand public communication web system with optical fibers, a semiconductor laser device and peripheral devices including a modulator should be improved in their characteristics and manufactured at a reasonable cost. Particularly, for a high-density communication with the optical devices, a modulator with a high modulation rate has been demanded to process a large amount of information. For this purpose, an external modulator has been widely used to modulate a laser beam continuously emitted from the semiconductor laser device in response to the electrical signals applied to the modulator. This is because the external modulator advantageously reduces the deviation of the laser beam wavelength, which might be caused in its modulation of the laser signal, allowing the beam signal to be transmitted over the long distance.
A reverse-biased electrical field is applied on the external modulator with a beam absorption layer. By this application, a beam absorption index of the beam absorption layer is changed according to the Franz-Keldysh effect or the Stark quantum confinement effect, so that the laser beam therethrough is absorbed more effectively. Thus, the laser beam can be switched on and off by the electrical signal applied to the modulator.
As described above, the higher density communication with the optical devices requires a higher modulation rate, which in turn requires an improved modulating frequency characteristics, i.e. an improved cut-off frequency. Basically, the modulating frequency characteristics depends upon its CR time constant which has parameters such as capacitance and resistance. Therefore, reduction should be made in the capacitance and/or the resistance of the modulator for the high-density optical communication.
Disadvantageously, the incorporation of the external modulator with the semiconductor laser device brings a difficulty in optical coupling between the semiconductor laser device and the modulator and results in an increase of the additional parts, making the device costly.
In order to overcome the problems, a modulator-integrated semiconductor laser device having a modulator monolithically integrated therewith on a common semiconductor substrate has been developed. However since the modulator-integrated semiconductor laser device has the frequency characteristics dependent on the CR time constant of the modulator, it is also critical to reduce the capacitance and/or resistance of the modulator for the high-density optical communication.
Specifically, descriptions will be made to the conventional modulator. Referring to FIGS. 11 and 12, the modulator 200 comprises a substrate 202 of n-type InP, a modulator portion 226, a pad portion 220, and a channel portion 228. In the following description, terminologies of xe2x80x9cn-typexe2x80x9d and xe2x80x9cp-typexe2x80x9d are referred to as xe2x80x9cn-xe2x80x9d and xe2x80x9cp-xe2x80x9d, respectively.
As clearly shown in FIG. 12, the modulator portion 226 comprises a lower cladding layer 204, a beam absorption layer 206, and a first upper cladding layer 208, successively formed on the substrate 202. The modulator portion 226 also comprises a current blocking layer 210 including a first lower embedded layer 210a made of InP doped with Fe, a hole-trapping layer 210b made of n-InP, and a second upper embedded layer 210c made of InP doped with Fe. The modulator portion 226 further comprises a second cladding layer 212 made of InP and a contact layer 214 made of p-InGaAs.
The pad portion 220 comprises a multi-layered structure which is similar to that of the current blocking layer 210, the second upper cladding layer 212, the contact layer 214, and the insulating layer 216.
An insulating layer 216 is disposed on the modulator portion 226, the pad portion 220, and the channel portion 228, leaving an elongated opening above and opposing to the beam absorption layer 206.
A wire layer indicated by reference numerals 220a, 222, and 218 is disposed on the pad portion 220 and the channel portion 228, and in contact with the contact layer 214 through the opening, and a back electrode 224 is formed on the back surface of the substrate 202.
Referring again to FIG. 11, the operation of the modulator 200 is described hereinafter. In general, the modulator 200 receives a laser beam L1 as indicated in FIG. 11 at its couple-in facet and delivers a laser beam L2 at its couple-out facet. In this regard, the reverse-biased voltage is applied between the back electrode 224 and the modulator electrode 218 through the pad electrode 220a, which changes the beam absorption index of the reverse-biased beam absorption layer 206 due to the Franz-Keldysh effect or the Stark quantum confinement effect. While the reverse-biased voltage is turned off, the laser beam L2 can be delivered. But contrary to this, the reverse-biased beam absorption layer 16 absorbs the laser beam L1, and then prevents the laser beam L2 from being delivered. This provides the ON-OFF switching of the laser beam L2 generating a high-rate modulating electrical signal. In this manner, the electrical signal can be transformed into the laser beam signal in the form of pulses.
As described above, the cut-off frequency of the modulator 200 depends upon the CR time constant. Then, in order to achieve a high-rate modulator for the high-density signal communication, the CR time constant, that is, the capacitance and/or the resistance of the modulator 200 should be reduced.
It should be noted that the capacitance of the modulator 200 equals to the sum of the capacitance of the modulator portion 226, the pad portion 220, and the channel portion 228. Disadvantageously, each of the capacitances of the modulator portion 226, the pad portion 220, and the channel portion 228 can not be readily reduced due to the structural reasons, which will be described below. Therefore, the present invention addresses to the reduction of the capacitance, especially of the channel portion 228 and the modulator portion 226, thereby increasing the cut-off frequency for the high-density optical communication.
JP8-172242, A, discloses an another conventional semiconductor laser device. The semiconductor laser device comprises a pad portion formed on the common substrate having the same layer structure as that of an active layer (the light emitting and waveguiding layer), and a wire layer formed on an insulating layer connecting an anode electrode of the active layer with a bonding pad. However, it fails to describe the layer structure beneath the insulating layer.
Also, JP6-216464, discloses an another example of a conventional field-absorption type modulator, in which a polyimide layer is deposited on the common substrate and adjacent to the ridge-like modulator. In this structure, although a buffer layer made of undoped InP is deposited beneath the modulator and the polyimide layer, no description is made to a channel for dividing the modulator from the polyimide layer.
Examples of a manufacturing process using an etching-inhibiting layer to form a beam waveguide structure are JP11-97799, A, JP11-87836, A, and JP7-231145, A.
A first object of the present invention is to provide a modulator in which its capacitance is reduced to increase the cut-off frequency and thereby to improve the frequency-characteristics.
A second object of the present invention is to provide a modulator-integrated laser device integrating a modulator therein, in which its capacitance is reduced to increase the cut-off frequency and thereby to improve the frequency-characteristics.
A third object of the present invention is to provide a process for manufacturing the modulator and the modulator-integrated semiconductor laser device integrating the modulator, in which its capacitance is reduced to increase the cut-off frequency and thereby to improve the frequency-characteristics.
The modulator according to the present invention comprises: a) a semiconductor substrate; b) a ridge-like modulator structure formed on the semiconductor substrate for modulating a laser beam, the modulator structure including, a ridge-like beam waveguide, a current blocking layer disposed on opposite sides of the beam waveguide and the substrate, the current blocking layer having a first semi-insulating semiconductor layer and a first semiconductor layer successively formed on the substrate, and c) a pad structure formed on the semiconductor substrate and spaced away from the modulator structure via a channel portion, for providing a wire-bonding pad base; d) the channel portion including the first semi-insulating semiconductor layer extending from the modulator structure; e) an insulating layer extending on the modulator structure, the pad structure, and the channel portion, the insulating layer having an opening on the modulator structure; and f) a wire layer disposed on said insulating layer, said wire layer being in contact with said modulator structure through the opening, wherein the capacitance of the channel portion is reduced since not only the insulating layer but also the first semi-insulating semiconductor layer are formed beneath the wire layer.
In the modulator of the present invention, the current blocking layer further has a second semi-insulating semiconductor layer formed on the first semiconductor layer.
Further in the modulator of the present invention, the pad structure includes a layer structure same as that of the current blocking layer, so that the pad structure can be formed readily, and simultaneously as the current blocking layer.
And the modulator according to the present invention further comprises: a second semiconductor layer interposed between the first semi-insulating semiconductor layer and the first semiconductor layer, wherein the second semiconductor layer being etched more slowly than the first semiconductor layer, so that the second semiconductor layer functioning as an etching-inhibiting layer readily removes the first semiconductor thoroughly by etching.
In the modulator of the present invention, the modulator structure includes a stripe-like contact layer connected to the wire layer, the contact layer having an width narrower than that of the ridge-like modulator structure, wherein the capacitance of the modulator portion is reduced.
Also, the modulator-integrated semiconductor laser device according to the present invention comprises: a) a semiconductor substrate; b) a ridge-like modulator structure formed on the semiconductor substrate for modulating a laser beam, the modulator portion including, a ridge-like beam waveguide, and a current blocking layer disposed on opposite sides of the beam waveguide and the substrate, the current blocking layer having a first semi-insulating semiconductor layer and a first semiconductor layer successively formed on the substrate; c) a pad structure formed on the semiconductor substrate and spaced away from the modulator structure via a channel portion, for providing a wire-bonding pad base; d) the channel portion including the first semi-insulating semiconductor layer extending from the modulator structure; e) an insulating layer disposed on the modulator structure, the pad structure, and the channel portion with an opening on the modulator structure; f) a wire layer disposed above the pad structure and the channel portion via the insulating layer and in contact with the modulator structure through the opening; and g) a semiconductor laser structure disposed on the substrate and adjacent to the modulator structure in a longitudinal direction thereof, the semiconductor laser structure including an active layer optically connected to the beam absorption layer, wherein the capacitance of the channel portion is reduced since not only the insulating layer but also the first semi-insulating semiconductor layer are formed beneath the wire layer.
In the modulator-integrated semiconductor laser device of the present invention, the current blocking layer further has a second semi-insulating semiconductor layer formed on the first semiconductor layer.
And, in the modulator-integrated semiconductor laser device of the present invention, the pad structure includes a layer structure same as that of the current blocking layer, so that the pad structure can be formed readily, and simultaneously as the current blocking layer.
And the modulator-integrated semiconductor laser device according to the present invention further comprises: a second semiconductor layer interposed between the first semi-insulating semiconductor layer and the first semiconductor layer, wherein the second semiconductor layer being etched more slowly than the first semiconductor layer, so that the second semiconductor layer functioning as an etching-inhibiting layer readily removes the first semiconductor thoroughly by etching.
In the modulator-integrated semiconductor laser device of the present invention, the modulator structure includes a stripe-like contact layer connected to the wire layer, the contact layer having an width narrower than that of the ridge-like modulator structure, wherein the capacitance of the modulator portion is reduced.
Furthermore, A process for manufacturing a modulator of the present invention, comprises: a) a first step for growing a lower cladding layer, a beam absorption layer, and an upper cladding layer on a semiconductor substrate, and etching the resultant layers with a first stripe-like mask until the substrate is exposed to form a ridge-like waveguide; b) a second step for growing a first semi-insulating semiconductor layer and a first semiconductor layer successively with the first mask on opposite sides of the beam waveguide and the substrate to form a current blocking layer; c) a third step for removing the stripe-like dielectric layer; d) a fourth step for etching the resultant layers with a second stripe-like mask covering, in part, the ridge-like waveguide and the current blocking layer adjacent to the sides of the waveguide until the first semi-insulating semiconductor layer is exposed to form a ridge-like modulator structure and a pad structure; e) a fifth step for forming an insulating layer with an opening above the waveguide; and f) a sixth step for forming a wire layer on the insulating layer and in contact with the modulator structure through the opening, whereby the modulator with the reduced capacitance of the channel portion is readily formed since not only the insulating layer but also the first semi-insulating semiconductor layer are formed beneath the wire layer.
In the process of the present invention, the second step including a sub-step for growing a second semi-insulating semiconductor layer with the first mask on the first semiconductor layer after growing the first semiconductor layer, whereby the modulator with the improved cut-off frequency can be readily formed.
Also, a process for manufacturing a modulator-integrated laser device, comprising: a) a first step for growing a lower cladding layer, a beam absorption layer, and an upper cladding layer on a semiconductor substrate, and etching the resultant layers with a first stripe-like mask until the substrate is exposed to form a ridge-like waveguide; b) a second step for growing a first semi-insulating semiconductor layer and a first semiconductor layer successively with the first mask on opposite sides of the beam waveguide and the substrate to form a current blocking layer; c) a third step for removing the stripe-like dielectric layer; d) a fourth step for etching the resultant layers with a second stripe-like mask covering, in part, the ridge-like waveguide and the current blocking layer adjacent to the sides of the waveguide until the first semi-insulating semiconductor layer is exposed to form a ridge-like modulator structure and a pad structure; e) a fifth step for forming an insulating layer with an opening above the waveguide; f) a sixth step for forming a wire layer on the insulating layer and in contact with the modulator structure through the opening; and g) a seventh step for forming a semiconductor laser on the semiconductor substrate and adjacent to the modulator structure in a longitudinal direction thereof, the semiconductor laser structure including an active layer optically connected to the beam absorption layer, whereby the modulator with the reduced capacitance of the channel portion is readily formed since not only the insulating layer but also the first semi-insulating semiconductor layer are formed beneath the wire layer.
In the process of the present invention, the second step including a sub-step for growing a second semi-insulating semiconductor layer with the first mask on the first semiconductor layer after growing the first semiconductor layer, whereby the modulator with the improved cut-off frequency can be readily formed.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the sprit and scope of the invention will become apparent to those skilled in the art from this detailed description.